Wearable fitness devices (i.e., activity trackers) are increasingly popular for monitoring everyday activity. Research suggests that long-term adherence to activity trackers is relevant for their positive effects on health. Thus, it is essential to understand the factors that foster usage intensity and long-term adherence. Based on first research regarding users’ motives for using activity trackers and self-determination theory, we examined usage motives as predictors for the current and estimated future usage intensity. In addition, we investigated the relation of usage motives and user diversity facets (affinity for technology interaction, geekism, and need for cognition). Results of an online study with N = 58 regular users of activity trackers indicated a substantial variation of users’ intrinsic/extrinsic motivation for using an activity tracker. Further, positive relationships between intrinsic motivation, autonomous regulation and usage intensity were found. Regarding user diversity, affinity for technology interaction and geekism predicted the intrinsic motivation whereas need for cognition did not. Our results imply that, in order to obtain possible beneficial health effects of a more intensive activity tracker usage, users’ intrinsic motivation and autonomy have to be supported.